HER2 / ErbB2 is one of the four members of the ErbB receptor family of transmembrane receptor-like tyrosine kinases (1). The kinase activity of ErbB2 can be activated without ligand if it is overexpressed, and by association with other ErbB proteins (2). Overexpression of ErbB2 is detected in almost 40% of human breast cancers (3). ErbB2 is one of the major targets for the treatment of breast cancer and other carcinomas. Autophosphorylation did not modulate receptor turnover. A Tyr----Phe substitution of ErbB-2 Tyr-877 homologous to pp60c-src Tyr-416 did not alter ErbB-2 biological and biochemical properties, thus excluding the possibility that phosphorylation of this residue, located in the kinase domain, modulates ErbB-2 gp185 catalytic function (4).